First ill go for the unlock question:
I'm almost possitive Intel made it pretty **** hard with software to do.
You see, the E6400 is really almost the same quality in parts and as well made as the E6300. The E6300 probably ran a few degrees hotter or so, so they downclocked it some more to make it SUPER stable.
Thing is with the right Mobo, ram, and cooling the E6300 can reach Speeds of 3.4ghz+
If you were to say grab a pretty cheap mobo, and some DDR2-677 ram you could OC it to about 2.5ghz on stock cooling easily and the chip would be stable as long as you have just decent cooling in the case.
E6300 w/ Arctic Freezer Pro 7 @ 3.6ghz 1.4Vcore
Antec 900 Case
EVGA 750i FTW Mobo
4x1GB of G.skill DDR2-800
2 8800GT in SLI
22" Samsung Syncmaster
1 250GB WD and 1 Seagate 7200.10 in RAID 0
Corsair 520HX 3DMark06 Score: 13900